chore(sonar): demote single-use field to local (S1450)

LinqExpressionVisitor._tableName was only assigned and read inside
VisitConstant immediately before assigning FromClause. Drop the field
entirely and assign FromClause directly from entityType.Name.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Strata.SqlTools.LinqToSql/Visitors/LinqExpressionVisitor.cs

@@ -13,7 +13,6 @@ public class LinqExpressionVisitor : ExpressionVisitor
     private readonly StringBuilder _orderByBuilder = new();
     private readonly List<string> _methodCalls = new();
     private bool _isInWhereClause;
-    private string? _tableName;
 
     /// <summary>
     /// Gets the SELECT clause extracted from the expression.
@@ -119,8 +118,7 @@ public class LinqExpressionVisitor : ExpressionVisitor
                 var entityType = node.Type.GetGenericArguments().FirstOrDefault();
                 if (entityType != null)
                 {
-                    _tableName = entityType.Name;
-                    FromClause = _tableName;
+                    FromClause = entityType.Name;
                 }
             }
         }